Electromagnetic radiation may couple to leads or cables connecting electrical components within a device or connecting one component or device to another device or to a power source. If the electromagnetic radiation coupling is excessive, the induced energy can interfere adversely with the intended operation of the component, device or associated devices. This electromagnetic susceptibility is referred to as electromagnetic interference ("EMI").
Commercial facilities and well-known procedures exist for testing electronic devices for EMI. Typically, EMI is determined as a result of a test for one or more of radiated emissions, conducted emissions, radiated susceptibility and conducted susceptibility. Such tests are conducted by placing the device under test inside a "screen room", also known as an R-F enclosure or a faraday cage, which completely shields the device under test from external ambient electromagnetic radiation. The device is then exposed to a controlled level of electromagnetic radiation, e.g., between 1 and 300 v/m, that is swept through a selected frequency range, e.g., from 10 Hz to 18 GHz. The device is then monitored or measured for electromagnetic susceptibility over the selected frequency range using probes, preferably probes that are transparent to the electromagnetic radiation, and an oscilloscope, spectrum analyzer or other display device. The test results are then compared to an appropriate predetermined reference standard to determine whether the device under test has a tolerable electromagnetic immunity or EMI susceptibility that requires correction.
EMI is determined to exist when the sweep test results deviate from the correct or desired performance characteristics by a determinable amount. This may occur in one or more frequency ranges of the sweep test. Once the frequency (or bandwidth) and magnitude of the EMI is known, an EMI filter can be designed or a commercially available EMI filter can be selected and used to suppress the EMI signals. If the EMI occurs in discrete frequency ranges, more than one EMI filter may be used. Known filters include both broadband filters, T-filters, and notch filters, for example, Butterworth notch filters, and mechanical type filters (rod, disc, and plate resonators), "ferrite beads", etc.
One of the problems with the known techniques is that the design or selection of an appropriate filter for suppressing EMI in a specific application has largely been a matter of trial and error. It requires an individual of considerable experience in the field to design or select a filter, which filter then must be installed in the device and sweep tested to determine whether the filter has adequately achieved the intended purpose. This results in a time consuming process which can take days, weeks or sometimes months to resolve. For sweep tests using controlled electromagnetic fields, it also requires substantial time in a screen room to test each variation of the EMI filter design. It costs between $1000 to $3000 per day to rent a screen room.
Another problem with the known techniques is that conventional sweep tests of the device under test provide information only for a measured resistance, and do not take into consideration or measure the imaginary portion of the actual complex impedance. Thus, the selection of an EMI filter design for the device under test is complicated by having to guess at the imaginary impedance component corresponding to the determined resistance, and then trying different design variations until a particular filter design works adequately.
Heretofore, there is no known commercial tool or technique for use in designing an EMI filter efficiently or for determining whether an EMI filter design will likely suppress the undesired EMI to about the appropriate specification limits, prior to sweep testing a selected EMI filter prototype installed in the device under test.
There thus remains a continuing need for improvements in designing or selecting an appropriate filter for suppressing EMI susceptibility in electrical devices and determining whether the selected design will achieve the desired suppression of EMI.